Blender 2.78c Cycles particles change color by age
Here's what's going on, from Blender Docs about Particle Info Node:


  This node currently only supports parent particles, info from child
  particles is not available.


So your only option is to turn off Particle Children and crank up Particle Number to make it 'similar'.

Also if you turn on Parents in Particle Render section some of them will be white but only a few.



I've checked also in Blender 2.79RC2 but it's not working there either.